Ceramic honeycomb structure

ABSTRACT

A ceramic honeycomb structure ( 1 ) constituted by cell walls (ribs) ( 2 ) forming a composite structure from a plurality of cells ( 3 ) being adjacent each other and a honeycomb outer wall ( 4 ) surrounding and holding the outermost peripheral cells located at the circumference of the composite structure; said composite structure satisfying the followings: 
     the basic thickness of the cell walls ( 2 ) (the basic cell wall thickness) (Tc) is Tc≦0.12 mm, the outer wall thickness (Ts) of the honeycomb structure is Ts≧0.05 mm, and the open frontal area (P) is P≧80%, and there is a relation shown by formula: 
     
       
         1.10≦( Tr   1   ˜Tr   3-20 )/ Tc≦3.00    
       
     
     between the basic cell wall thickness (Tc) and each cell wall thickness (Tr 1 ˜Tr 3-20 ) of cells existing between an outermost peripheral cell and any cell within a first end cell from a third cell to a twentieth cell extending inwardly, taking the outermost peripheral cell as a first starting cell.

The present invention relates to a ceramic honeycomb structure. Moreparticularly, the present invention relates to a ceramic honeycombstructure capable of balancing the disadvantages incurred by theincreased pressure loss and the decreased thermal shock resistanceagainst the advantages brought about by the increased isostatic strengthand the cell wall shape and honeycomb external shape of higher accuracy,and which is suitably used, for example, as a carrier for catalyst forautomobile exhaust gas purification. The ceramic honeycomb structure ofthe present invention is also used suitably as a filter for dieselparticulates or the like, as a chemical reactor (e.g. a catalyst carrierfor fuel cell reformer), or as a heat exchanger.

BACKGROUND ART

As catalysts for purification of automobile exhaust gas, there are usedso-called honeycomb catalysts wherein a catalyst component is loaded onthe surfaces of the cell walls of a ceramic honeycomb carrier (honeycombstructure). In these catalysts, since the axial direction strength ofthe honeycomb carrier is higher than its strength in the sectional(diameter) direction, the honeycomb carrier was held in the axialdirection. In this holding manner, in order to prevent the breakage ofthe honeycomb carrier occurring at around the periphery of its ends inaxial direction holding, the thickness of the cell walls (ribs) near thecircumference of the honeycomb carrier was made larger than thethickness of the cell walls in the inner portion of the honeycombcarrier to increase the anti-pressure strength of the honeycomb carrierin the axial direction.

Recently, however, the higher output adopted in engines has required alower pressure loss of honeycomb catalyst and the stricter regulationemployed for exhaust gas has needed the effective utilization of wholecatalyst carrier; therefore, it has been started to hold the honeycombcarrier mainly at its outer surface, in place of holding in the axialdirection. Another reason for this is that the stricter regulation forexhaust gas has invited a larger catalyst volume and an increasedcatalyst mass and, as a result, the holding in the axial direction hasbecome unable to give a sufficient holding area and promise sufficientholding relative to engine vibration.

Meanwhile, in order to enhance the purification ability of catalyst, ithas been started to make thinner the thickness of the cell walls of ahoneycomb carrier and decrease the weight of the honeycomb carrier andthereby reduce the heat capacity of a catalyst and enhance itspurification ability (warm-up property).

The above use of cell walls of thinner thickness tends to result in alower fracture strength against the external pressure which thehoneycomb carrier receives at the outer surface.

In order to meet the recent even stricter regulation for exhaust gas, ithas been aimed to improve the conditions of engine combustion and thepurification ability of catalyst. As a result, the temperature ofexhaust gas has become higher year by year and the thermal shockresistance required for a honeycomb carrier has become stricter.

Thus, due to the thinning of the cell walls, the employment of holdinghoneycomb carrier at the outer surface, the increase in temperature ofexhaust gas and the like, the setting of cell wall thickness andhoneycomb outer wall thickness, the increase in the isostatic strengthof honeycomb structure, and the higher accuracies of honeycomb externalshape and cell wall shape have become important tasks to be achieved.

In connection with the above, there was proposed, in JP-A-54-110189, ahoneycomb carrier structure whose cell walls are made thinner at a givenratio from the outermost peripheral cell wall towards the center of thecross-section. In this structure, since use of a thin wall in the entirehoneycomb carrier is impossible, the total mass of the honeycomb carrieris inevitably large, posing a problem in the warm-up property of thehoneycomb carrier. This structure is undesirable also in pressure loss.

There was also proposed, in JP-A-54-150406 and JP-A-55-147154, astructure wherein the walls of the cells near the circumference of thestructure are made thicker than those of the inner cells. However, nomention is made on the thickness of the outer wall or on the specificrelation between different cell wall thickness therein.

In these honeycomb structures of the prior art, since the thickness ofinner cell walls is as large as 0.15 mm or more and the holding is madein the axial direction, the thickness of the honeycomb outer wall wasnot a problem. One may merely point out that too large an outer wallthickness gives a low thermal shock resistance, if forced to do so.

Further in WO 98/05602 was proposed a ceramic honeycomb structurewherein the average cell wall thickness T is 0.05 to 0.13 mm, theaverage outer wall thickness is larger than T, W>T (W is an averagewidth of contact between cell wall and outer wall), and 0.7≧−(T/4)+0.18.

This ceramic honeycomb structure exhibits some effect in prevention ofperipheral chipping during handling; however, it was not fullysatisfactory in increased pressure loss, reduced thermal shockresistance, increase in isostatic strength, and the improvements in theaccuracies of cell wall shape and honeycomb structure external shape.

No in-depth investigation has hitherto been made particularly on theimprovements in the accuracies of cell wall shape and honeycombstructure external shape. That is, a ceramic honeycomb structure isgenerally molded by extruding, for example, a cordierite raw materialfor ceramic through a die having lattice-shaped slits; then dried; andfired to become a product. When a smaller cell wall thickness isemployed, the cell walls tend to deform during molding, owing to thecause mentioned later and resultantly the fired material obtained showedno satisfactory isostatic strength while this did not happen when thecell wall thickness was as large as 0.15 mm or more. Nevertheless, nosufficient investigation has been made. The deformed cell walls aredestroyed at the deformed sites by a small force. That is, when cellwalls do not deform and are molded at a high accuracy, theytheoretically become sites of compression stress when a pressure isapplied to the outer surface of honeycomb structure, and the destructionof honeycomb structure takes place owing to the buckling of cell wall orouter wall. Meanwhile, when cell walls have deformed, a bending stress(a stress in tensile direction) is generated at the deformed sites,resulting in easy destruction. In general, materials are less resistantto tensile strength than to compression stress and, in ceramicmaterials, in particular, the ratio (about 1/10) of tensile strength tocompression strength is very small as compared with that (about 1/3) ofmetal materials. Therefore, when there is deformation of cell walls,destruction takes place at a strength considerably lower than a strengthat which destruction takes place ordinarily.

The present invention has bee made in view of the above problems, andaims at providing a ceramic honeycomb structure capable of balancing thedisadvantages incurred by the increased pressure loss and the decreasedthermal shock resistance against the advantages brought about by theincreased isostatic strength and the cell wall shape and honeycombexternal shape of higher accuracy, and which is suitable particularlyas, for example, a carrier for catalyst for automobile exhaust gaspurification.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, the present inventor made a studyincluding various tests mentioned later, with considering the thinnercell walls recently employed in honeycomb carriers. As a result, thefollowing was found out. That is, the adoption of a thick wall only inthe cells near the circumference of honeycomb structure as seen in theprior art is insufficient and attention must be paid also to theextrudability of honeycomb structure; therefore, the designing of ahoneycomb structure need be made while paying attention not only to therelation between the wall thickness of each outermost peripheral celland the wall thickness of inner cells (basic cells), i.e. the basic cellwall thickness, but also, while considering the basic cell wallthickness and the thickness of honeycomb outer wall, to the relationbetween the basic cell wall thickness and the wall thickness of thecells existing between the outermost peripheral cell taken as startingcell and any cell taken as end cell of a certain number of cellsextending inwardly from the starting cell and located near thecircumference of honeycomb structure; by making the designing of ahoneycomb structure as above, the above-mentioned aim of the presentinvention can be achieved. The present invention has been completedbased on the above finding.

It has heretofore been believed that a honeycomb structure having a highstrength against a pressure applied to the outer surface can be obtainedby allowing the honeycomb structure to have an increased outer wallthickness. There were produced cordierite-made thin wall honeycombstructure samples having an outer diameter of 90 mm, a length of 110 mm,a square cell shape, a cell wall thickness of 0.11 mm and a cell densityof 9.3×10⁵ cells/m² (wall-to-wall distance: 1.04 mm), with varying theouter wall thickness between 0.1 and 0.9 mm; and they were measured forisostatic strength and the results are shown in FIG. 5. As shown in FIG.5, the isostatic strength showed no increase and reversely decreased,even if the outer wall thickness was made thicker than 0.4 mm.

The reason for the fact that mere increase in outer wall thickness givesno increase in isostatic strength, is considered to be that as the outerwall thickness is increased, the amount of shape deformation of the wall(rib) of the cells near the circumference of honeycomb structureincreases and moreover the number of the deformed walls increases. Thisis considered to be because as the outer wall thickness is increased,the amount of the raw material passing, during extrusion molding,through the die slit for outer wall formation increases and, as aresult, the ribs of the cells near the circumference of honeycombstructure are dragged toward the outer wall and the raw material flow inouter wall and the raw material flow in ribs become unbalanced. The mainreasons therefor are considered to be that the change of ribs to smallerthickness incurs easy buckling deformation and that, when the honeycombstructure after extrusion molding is fixed by a jig at the outersurface, the honeycomb structure per se deforms owing to the own weightand, as a result, deformation of the outer wall and the inside ribs,particularly the ribs near the circumference of honeycomb structuretakes place. This tendency is considered to be higher as the ribs arethinner and the structure is bigger.

According to strength of materials, buckling strength is basicallyproportional to the square of cell wall (rib) thickness, as shown by thefollowing formula. It is appreciated from the formula that the thicknessof cell wall has a great influence on the strength of honeycomb carrier.

Buckling strength=(kπ ² E)×(t/L)²

wherein k is a coefficient, E is a Young s modulus, L is a cell walllength, and t is a cell wall thickness.

Also, there was conducted a test for thermal shock resistance insupercooling, wherein a honeycomb carrier having a cell wall thicknessof 0.11 mm was heated in an electric furnace for a given length of timeto make the temperature uniform and then taken out from the furnace. Theresults are shown in FIG. 6. As the outer wall thickness of thehoneycomb carrier was increased, the thermal shock resistance decreasedand the decrease was larger when the outer wall thickness was 0.7 mm ormore. This is considered to be that as the outer wall is thicker, theouter wall per se has a larger heat capacity and the temperaturedifference between inside and outside the outer wall is bigger.

For lowering the heat capacity of outer wall, there is an idea offorming notches in the outer wall, as seen in the above-mentionedJP-A-54-150406. This idea has a meaning if the outer wall issufficiently thick; however, when the cell wall thickness is very thin(1.12 mm or less), the outer wall cannot be freely made thicker asmentioned previously and resultantly the effect of notches is small.Conversely, there is a risk of decreasing the rigidity of the outerwall.

Also, strength was measured in honeycomb carriers wherein the wallthickness in the cells near the circumference of the carriers were madelarger than the inner cell (basic cell) wall thickness as in the priorart. As a result, there was certainly an increase in strength; however,as the wall thickness in the cells near the circumference of thecarriers was too large, the strength tended to decrease. Inspection of acarrier having a considerably large wall thickness in the outermostperipheral cell indicated that the cell wall of the outermost peripheralcell had deformation. The reason is considered to be the same as thereason for the above-mentioned fact that mere increase in outer wallthickness gives no increase in isostatic strength. As in the case ofmere increase in outer wall thickness, mere increase in the wallthickness in the cells near the circumference of carrier does notnecessarily contribute to the increase in isostatic strength.

FIG. 7 shows the results obtained when each cell wall thickness of thecells existing between each outermost peripheral cell taken as astarting cell and any cell of the 2nd to 20th cells extending inwardlyfrom the starting cell was increased from the basic cell wall thickness(75 μm) to 100 μm, 150 μm or 200 μm and each of the resulting honeycombstructures was measured for isostatic strength (%). As can be taken fromFIG. 7, the degree of increase in isostatic strength was low when thewall thickness of the first to 4th cells was made larger; a strikingincrease in isostatic strength was seen when the wall thickness of eachcell from any of the 5th to 15th cells was made larger; and there wasobserved the settlement in the degree of increase in isostatic strengtheven if the wall thickness of the 5th to 20th cells was made larger. Noincrease in isostatic strength was seen when the wall thickness of the1st to 2nd cells was made larger; however, a sign of increase was seenwhen the wall thickness of the 1st to 3rd or 4th cells was made larger;and a clear increase was seen when the wall thickness of 5th or latercells was made larger.

Strength measurement was also made by changing the dimension of externalshape of honeycomb structure. In a honeycomb structure having a circularsection of 144 mm or more in diameter or having an elliptical sectionhaving the same sectional area as the circular section, an increase instrength was seen when the wall thickness of each cell of the 10th to30th cells was made larger; and there was observed in a similar tendencyto that mentioned above in the degree of increase in strength when thewall thickness of the 10th to 40th cells was made larger. (This was thesame tendency as seen above.)

FIG. 8 shows the results obtained when each cell wall thickness(Tr₁˜Tr₁₃) of the outermost peripheral cell, taken as a starting cell tothe 13th cell was made larger one by one than the basic cell wallthickness (Tc) so as to give a ratio of 1.00 to 3.00 in terms of[(Tr₁˜Tr₁₃)/(Tc)] and measurement of isostatic strength (%) was made. Asseen from FIG. 8, a sharp increase in isostatic strength was seen fromwhen the ratio of said each cell wall thickness to basic cell wallthickness, i.e. [(Tr₁˜Tr₁₃)/(Tc)] was 1.10; and the degree of theincrease settled when the [(Tr₁˜Tr₁₃)/(Tc)] was 2.5.

FIG. 9 is a partly enlarged view of FIG. 8. As seen from FIG. 9, sincethe sharp increase in isostatic strength lasts to [(Tr₁˜Tr₁₃)/(Tc)]=1.20, it is preferred to adopt a [(Tr₁˜Tr₁₃)/(Tc)] of 1.20 or more.

FIG. 10 shows the results obtained when each cell wall thickness(Tr₁—Tr₁₃) of the 1st cell (taken as starting cell) to the 13th cell wasmade larger one by one than the basic cell wall thickness (Tc) so as togive a ratio of 1.00 to 3.00 in terms of [(Tr₁˜Tr₁₃)/(Tc)] andmeasurement of pressure loss (%) was made. As seen from FIG. 10, a sharpincrease was seen from when the [(Tr₁˜Tr₁₃)/(Tc)] was 3.00; therefore,it is preferred to adopt ordinarily a [(Tr₁˜Tr₁₃)/(Tc)] of 3.00 or lessand, from practical viewpoints, a [(Tr₁˜Tr₁₃)/(Tc)] of 2.50 or less,preferably 1.60 or less.

FIG. 11 shows the results obtained when each cell wall thickness(Tr₁˜Tr₁₅) of the outermost peripheral cell taken as a starting cell tothe 15th cell was made thicker than the basic cell wall thickness (Tc)so as to give a ratio of 2.0 in terms of [(Tr₁˜Tr₁₅)/(Tc)] andmeasurement of pressure loss (%) was made (case 1); when, in addition tothe conditions of the case 1, each cell wall of the 16th to 20th cellswas allowed to have a sectional shape of an inverse trapezoid (the minorbase was present inwardly), the thickness of said each cell wall wasmade thinner as said each cell wall was located more inwardly, and thethinnest wall thickness was made identical to the basic cell wallthickness (Tc) (case 2); when, in addition to the conditions of the case1, each cell wall of the 16th to 20th cells was allowed to have asectional shape of a spool, the thickness of said each cell wall wasmade thinner as said each cell wall was located more inwardly, and thethinnest wall thickness was made identical to the basic cell wallthickness (Tc) (case 3); when the ratio of the outermost peripheral cellwall thickness (Tr₁) to the basic cell wall thickness (Tc) was set at2.0, each cell wall of the 2nd and later cells was allowed to have asectional shape of an inverse trapezoid (the minor base was presentinwardly), the thickness of said each cell wall was made thinner as saideach cell wall was located more inwardly, the smallest wall thicknesswas made identical to the basic cell wall thickness, and measurement ofpressure loss (%) was made (case 4); and when the ratio of the outermostperipheral cell wall thickness (Tr₁) to the basic cell wall thickness(Tc) was set at 2.0, each cell wall of the 2nd and later cells wasallowed to have a sectional shape of a spool, the thickness of said eachcell wall was made smaller as said each cell wall was located moreinwardly, the thinnest wall thickness was made identical to the basiccell wall thickness, and measurement of pressure loss (%) was made (case5). As seen from FIG. 11, pressure loss is large in the cases 1 to 3;therefore, when increase in pressure loss is a disadvantage, it ispreferred to make gradually smaller the cell wall thickness from theoutermost peripheral cell toward inner cells as in the cases 4 and 5.

FIG. 12 shows the results obtained when, in the cases of FIG. 11,thermal shock resistance (%) was measured in place of pressure loss. Asseen from FIG. 12, when the cell wall thickness was made graduallysmaller from the predetermined cell to a particular inner cell as in thecases 2 to 5, an increase in thermal shock resistance can be obtained ascompared with the case 1.

FIG. 13 shows the results obtained when pressure loss (%) was measuredby setting, at 2.0, the ratio of each cell wall thickness (Tr₁˜Tr₃₀) ofthe cells existing between the outermost peripheral cell taken as astarting cell and any cell extending therefrom to the 30th cell, to thebasic cell wall thickness (Tc), i.e. [(Tr₁˜Tr₃₀)/(Tc)]. As seen fromFIG. 13, pressure loss increases from when the number of cells ofthickened wall exceeds 20.

FIG. 14 shows the results obtained when external shape accuracy (mm) wasmeasured by setting one by one, at 1.6, the ratio of each cell wallthickness (Tr₁˜Tr₂₀) of the cells existing between the outermostperipheral cell taken as a starting cell and any cell extendingtherefrom to the 20th cell, to the basic cell wall thickness (Tc), i.e.[(Tr₁˜Tr₂₀)/(Tc)]. As seen from FIG. 14, external shape accuracy(dimensional accuracy) increases from when the number of cells ofthickened wall exceeds 5 and, when the cell walls of up to the 15thcells are made thicker, the dimensional accuracy is half of when thecell wall thickness is constant and the same as the basic cell wallthickness. The reason is considered to be that the thicker wallthickness adopted in the cells near the circumference of honeycombstructure increased the rigidity of the structure and the deformationoccurring from structure molding to its firing was suppressed. It isconsidered that this also contributes to the improvement in the uniformmolding.

Based on the results of the above study, there is provided the followingceramic honeycomb structure according to the present invention.

1A ceramic honeycomb structure (1) constituted by cell walls (ribs) (2)forming a composite structure from a plurality of cells (3) beingadjacent each other and a honeycomb outer wall (4) surrounding andholding the outermost peripheral cells located at the circumference ofthe composite structure;

characterized in that a basic thickness of cell walls (2) (the basiccell wall thickness) (Tc) is Tc≦0.12 mm, an outer wall thickness (Ts) ofthe honeycomb structure is Ts≧0.05 mm, and an open frontal area (P) isP≧80%, and there is a relation shown by a formula:

1.10≦(Tr ₁ ˜Tr ₃₋₂₀)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₂₀) of cells existing between an outermost peripheral cell andany cell within a first end cell from a third cell to a twentieth cellextending inwardly, taking the outermost peripheral cell as a firststarting cell.

[2] A ceramic honeycomb structure according to the above [1] whereinthere is a relation shown by a formula:

1.10≦(Tr ₁ ˜Tr ₃₋₁₅)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₁₅) of cells existing between an outermost peripheral cell andany cell within a first end cell from a third cell to a fifteenth cellextending inwardly, taking the outermost peripheral cell as a firststarting cell.

[3] A ceramic honeycomb structure according to the above [1] or [2],wherein any cell within a second end cell from a third cell to a fifthcell extending inwardly, taking a cell adjacent to the first end cellbut located inward therefrom as a second starting cell, has such a cellwall thickness that a section of said each cell wall has a rectangularshape whose minor side of rectangle is a cell wall thickness thereofwhen the honeycomb structure is cut by a plane perpendicular to thedirection of the cells (passages),and

a cell wall thickness having a shortest minor side is identical to thebasic cell wall thickness (Tc), by shortening a minor side thereof oneby one as a cell is located more inwardly.

[4] A ceramic honeycomb structure according to the above [1] or [2],wherein any cell within a second end cell from a third cell to a fifthcell extending inwardly, taking a cell adjacent to the first end cellbut located inward therefrom as a second starting cell, has such a cellwall thickness that a section of said each cell wall has such an inversetrapezoidal shape as a minor base of inverse trapezoid is a thickness ofsaid each cell wall when the honeycomb structure is cut by a planeperpendicular to the direction of the cells (passages), and

a thickness of a cell wall having a shortest minor base is identical tothe basic cell wall thickness (Tc), by shortening a minor base ofinverse trapezoid thereof one by one as said each cell wall is locatedmore inwardly.

[5] A ceramic honeycomb structure according to the above [1] or [2],wherein any cell within a second end cell from a third cell to a fifthcell extending inwardly, taking a cell adjacent to the first end cellbut located inward therefrom as a second starting cell, has such a cellwall thickness that a section of said each cell wall has such a spoolshape as an inner side of spool is shorter than an outer side when thehoneycomb structure is cut by a plane perpendicular to the direction ofthe cells (passages), and

a thickness of a cell wall having an shortest inner side is identical tothe basic cell wall thickness (Tc), by shortening inner side of spoolthereof one by one as said each cell wall is located more inwardly.

[6] A ceramic honeycomb structure according to the above [1], whereinthere is a relation shown by a formula

1.10≦Tr ₁ /Tc≦3.00

between the cell wall thickness (Tr₁) of each outermost peripheral celland the basic cell wall thickness (Tc), there is a relation shown by aformula

1.10≦(Tr ₁ ˜Tr ₃₋₂₀)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₂₀) within a third end cell from a third cell to a twentiethcell extending inwardly, taking the outermost peripheral cell as a firststarting cell,

a section of said each cell wall has such a rectangular shape as a minorside thereof is thickness of said each cell wall, or such an inversetrapezoidal shape as a minor base of inverse trapezoid is presentinwardly and is thickness of said each cell wall, or such a spool shapeas inner side of spool is shorter than outer side when the honeycombstructure is cut by a plane perpendicular to the direction of the cells(passages); and

a thickness of the cell wall having a shortest minor side, or a shortestminor base or a shortest inner side is identical to the basic cell wallthickness (Tc), by shortening the minor side of rectangle, or the inwardminor base of inverse trapezoid or the inner side of spool one by one assaid each cell wall is located more inwardly.

[7] A ceramic honeycomb structure according to any of the above [1] to[6], wherein there is the following relation

1.10≦(Tr ₁ ˜Tr ₃₋₂₀)/Tc≦2.50

between the basic cell wall thickness (Tc) and said each cell wallthickness (Tr₁˜Tr₃₋₂₀).

[8] A ceramic honeycomb structure according to any of the above [1] to[6], wherein there is the following relation

1.20≦(Tr ₁˜Tr₃₋₂₀)/Tc≧1.60

between the basic cell wall thickness (Tc) and said each cell wallthickness (Tr₁˜Tr₃₋₂₀).

[9] A ceramic honeycomb structure according to any of the above [1] to[8], wherein the cells have a sectional shape of a triangle or a higherpolygon.

[10] A ceramic honeycomb structure according to any of the above [1] to[9], wherein the honeycomb outer wall has a sectional shape of a circle,an ellipse, a trapezoid, a triangle, a tetragon, a hexagon or a specialshape whose left and right are asymmetrical to each other.

[11] A ceramic honeycomb structure according to any of the above [1] to[10], wherein the honeycomb outer wall has a diameter of 144 mm or morewhen it has a circular sectional shape and, when it has a sectionalshape other than a circular sectional shape, it has a sectional areaequal to when it has a circular sectional shape, and,

there is the following relation

1.10≦(Tr ₁ ˜Tr ₁₀₋₄₀)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₁₀₋₄₀) of cells existing within a first end cell from a thirdcell to a fortieth cell extending inwardly, taking the outermostperipheral cell as a first starting cell.

[12] A ceramic honeycomb structure according to any of the above [1] to[10], wherein the honeycomb outer wall has a diameter of 144 mm or morewhen it has a circular sectional shape and, when it has other than acircular sectional shape, it has a sectional area equal to when it has acircular sectional shape, and there is a following relation shown by aformula:

1.10≦(Tr ₁ ˜Tr ₁₀₋₃₀)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₁₀₋₃₀) of cells within a first starting end cell from a tenthcell to a thirtieth cells extending inwardly, taking the outermostperipheral cell as a first starting cell.

[13] A ceramic honeycomb structure according to any of the above [1] to[12], wherein the cell walls and the honeycomb outer wall are made of atleast one kind of materials selected from the group consisting ofcordierite, alumina, mullite, silicon nitride, aluminum titanate (AT),zirconia and silicon carbide.

[14] A ceramic honeycomb structure according to any of the above [1] to[13], which is used as a carrier for catalyst for automobile exhaust gaspurification.

[15] A ceramic honeycomb structure according to any of the above [1] to[14], which is assembled into a catalytic converter by loading acatalyst component on the cell walls and holding the honeycomb outerwall at the outer surface.

[16] A ceramic honeycomb structure according to any of the above [1] to[15], wherein the corners of each cell are formed so as to have a radiusof curvature of 1.2 mm or less.

[17] A ceramic honeycomb structure according to any of the above [1] to[16], wherein each intersection between each outermost peripheral cellwall and the honeycomb outer wall is formed so as to have a radius ofcurvature of 1.2 mm or less.

[18] A ceramic honeycomb structure according to any of the above [1] to[17], wherein there is cell deformation and, when a diameter of thehoneycomb structure is 120 mm or less, a first or third end cell is anyof a third cell to a fifth cell and, when a diameter is more than 120mm, a first or a third end cell is any of a sixth cell to a twentiethcell.

[19] A ceramic honeycomb structure according to any of the above [1] to[18], wherein there is provided with a corrugated cell wall having acorrugation in the direction of the cells (passages) between at leastone pair of cells adjacent to each other, of the cells from the firststarting cell to the first end cell or from the second starting cell tothe second end cell or from the third starting cell to the third endcell.

As described above, the present invention can provide a ceramichoneycomb structure wherein the disadvantages incurred by the increasedpressure loss and the decreased thermal shock resistance and theadvantages brought about by the increased isostatic strength and thecell wall shape and honeycomb external shape of higher accuracy arebalanced appropriately and which is suitably used, for example, as acarrier for catalyst for automobile exhaust gas purification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view schematically showing an example of theceramic honeycomb structure of the present invention. FIG. 1(b) is aplan view schematically showing an example of the ceramic honeycombstructure of the present invention.

FIG. 2(a) is a partly enlarged view of the portion A of FIG. 1(b). FIG.2(b) is a further enlarged view of FIG. 2(a).

FIG. 3(a) is a sectional view schematically showing an example accordingto the ceramic honeycomb structure of the present invention, wherein anycell within a second end cell from a third cell to a fifth cellextending inwardly, taking a cell adjacent to the first end cell butlocated inward therefrom as a second starting cell, has such a cell wallthickness that a section of said each cell wall has such an inversetrapezoidal shape as a minor base of inverse trapezoid is a thickness ofsaid each cell wall when the honeycomb structure is cut by a planeperpendicular to the direction of the cells (passages), and

a thickness of a cell wall having a shortest minor base is identical tothe basic cell wall thickness (Tc), by shortening a minor base ofinverse trapezoid thereof one by one as said each cell wall is locatedmore inwardly. FIG. 3(b) is a sectional view schematically showing anexample according to the ceramic honeycomb structure of the presentinvention, wherein any cell within a second end cell from a third cellto a fifth cell extending inwardly, taking a cell adjacent to the firstend cell but located inward therefrom as a second starting cell, hassuch a cell wall thickness that a section of said each cell wall hassuch a spool shape as an inner side of spool is shorter than an outerside when the honeycomb structure is cut by a plane perpendicular to thedirection of the cells (passages), and

a thickness of a cell wall having an shortest inner side is identical tothe basic cell wall thickness (Tc), by shortening inner side of spoolthereof one by one as said each cell wall is located more inwardly. FIG.3(c) is a sectional view schematically showing an example according tothe ceramic honeycomb structure of the present invention, wherein anycell within a second end cell from a third cell to a fifth cellextending inwardly, taking a cell adjacent to the first end cell butlocated inward therefrom as a second starting cell, has such a cell wallthickness that a section of said each cell wall has a rectangular shapewhose minor side of rectangle is a cell wall thickness thereof when thehoneycomb structure is cut by a plane perpendicular to the direction ofthe cells (passages),and

a cell wall thickness having a shortest minor side is identical to thebasic cell wall thickness (Tc), by shortening a minor side thereof oneby one as a cell is located more inwardly.

FIG. 4 is a drawing schematically showing a case where the ceramichoneycomb structure (honeycomb carrier) of the present invention hasbeen accommodated in a catalytic converter container.

FIG. 5 is a graph showing the results obtained when there were producedcordierite-made thin wall honeycomb structure samples having an outerdiameter of 90 mm, a length of 110 mm, a square cell shape, a cell wallthickness of 0.11 mm and a cell density of 600 cpsi (wall-to-walldistance: 1.04 mm) wherein the outer wall thickness was varied between0.1 and 0.9 mm to measure isostatic strength thereof.

FIG. 6 is a graph showing the results obtained when there was conducteda test for thermal shock resistance in supercooling, wherein a honeycombcarrier having a cell wall thickness of 0.11 mm was heated in anelectric furnace for a given length of time to make the temperatureuniform and then taken out from the furnace.

FIG. 7 is a graph showing the results obtained when each cell wallthickness of the cells existing between each outermost peripheral celltaken as a starting cell and any cell of the 2nd to 20th cells extendinginwardly from the starting cell was increased from the basic cell wallthickness (75 μm) to 100 μm, 150 μm or 200 μm and each of the resultinghoneycomb structures was measured for isostatic strength (%).

FIG. 8 is a graph showing the results obtained when each cell wallthickness (Tr₁˜Tr₁₃) of the 1st cell (taken as starting cell) to the13th cell was made thicker one by one than the basic cell wall thickness(Tc) so as to give a ratio of 1.00 to 3.00 in terms of [(Tr₁˜Tr₁₃)/(Tc)]and measurement of isostatic strength (%) was made.

FIG. 9 is a partly enlarged view of FIG. 8.

FIG. 10 is a graph showing the results obtained when each cell wallthickness (Tr₁˜Tr₁₃) of the 1st cell (taken as starting cell) to the13th cell was made thicker one by one than the basic cell wall thickness(Tc) so as to give a ratio of 1.00 to 3.00 in terms of [(Tr₁˜Tr₁₃)/(Tc)]and measurement of pressure loss (%) was made.

FIG. 11 is a graph showing the results obtained when each cell wallthickness (Tr₁˜Tr₁₅) of the 1st cell (taken as starting cell) to the15th cell was made larger than the basic cell wall thickness (Tc) so asto give a ratio of 2.0 in terms of [(Tr₁˜Tr₁₅)/(Tc)] and measurement ofpressure loss (%) was made (case 1); when, in addition to the conditionsof the case 1, the section of each cell wall of the 16th to 20th cellswhen cut by a plane perpendicular to the cell (passage) direction wasallowed to have such an inverse trapezoidal shape as the minor base ofinverse trapezoid was present inwardly and was the thickness of saideach cell wall, the minor base of inverse trapezoid was made shorter assaid each cell wall was more inward, and the thickness of the cell wallhaving the shortest minor base was made identical to the basic cell wallthickness (Tc) (case 2); when, in addition to the case 1, the section ofeach cell wall of the 16th to 20th cells when cut by a planeperpendicular to the direction of the cell (passage) direction wasallowed to have such a spool shape as the inner side of spool wasshorter than the outer side and was the thickness of said each cellwall, the inner side of spool was made shorter as said each cell wallwas more inward, and the thickness of the cell wall having the shortestinner side was made identical to the basic cell wall thickness (Tc)(case 3); when the ratio [Tr₁/(Tc)] of the outermost peripheral cellwall thickness (Tr₁) to the basic cell wall thickness (Tc) was set at2.0, the section of each cell wall of 2nd and later cells when cut by aplane perpendicular to the cell (passage) direction was allowed to havesuch an inverse trapezoidal shape as the minor base of inverse trapezoidwas present inwardly and was the thickness of said each cell wall, theminor base of inverse trapezoid was made shorter as said each cell wallwas more inward, the thickness of the cell wall having the shortestminor base was made identical to the basic cell wall thickness (Tc), andpressure loss (%) was measured (case 4); and when the ratio [Tr₁/(Tc)]of the outermost peripheral cell wall thickness (Tr₁) to the basic cellwall thickness (Tc) was set at 2.0, the section of each cell wall of 2ndand later cells when cut by a plane perpendicular to the cell (passage)direction was allowed to have such a spool shape as the inner side ofspool was shorter than the outer side and was the thickness of said eachcell wall, the inner side of spool was made shorter as said each cellwall was more inward, the thickness of the cell wall having the shortestinner side was made identical to the basic cell wall thickness (Tc), andpressure loss (%) was measured.

FIG. 12 is a graph showing the results obtained when, in the cases ofFIG. 11, thermal shock resistance (%) was measured in place of pressureloss.

FIG. 13 is a graph showing the results obtained when pressure loss (%)was measured by setting, at 2.0, the ratio of each cell wall thickness(Tr₁˜Tr₃₀) of the cells existing between the outermost peripheral celltaken as a starting cell and any cell extending therefrom to the 30thcell, to the basic cell wall thickness, i.e. [(Tr₁˜Tr₃₀)/(Tc)].

FIG. 14 is a graph showing the results obtained when external shapeaccuracy (mm) was measured by setting, at 1.6, the ratio of each cellwall thickness (Tr₁˜Tr₂₀) of the cells existing between the outermostperipheral cell taken as a starting cell and any cell extendingtherefrom to the 20th cell, to the basic cell wall thickness, i.e.[(Tr₁˜Tr₂₀)/(Tc)].

FIG. 15 is a drawing schematically showing an idea of cell deformation.

FIG. 16 is a drawing showing a relation between the diameter of ceramichoneycomb structure and strength increase.

FIG. 17 is a perspective view schematically showing a corrugated cellwall having a corrugation in the cell (passage) direction.

FIG. 18 is a drawing schematically showing the tester used inmeasurement of isostatic strength.

FIG. 19 is a graph showing the cold-hot cycle of 1,200° C.×cycles usedin the test method for measurement of isostatic strength.

BEST MODE FOR CARRYING OUT THE INVENTION

Specific description is made below on the best mode for carrying out thepresent invention.

As described previously, the ceramic honeycomb structure of the presentinvention is constituted by cell walls (ribs) forming a compositestructure from a plurality of cells being adjacent each other and ahoneycomb outer wall surrounding and holding the outermost peripheralcells located at the circumference of the composite structure;

characterized in that a basic thickness of cell walls (the basic cellwall thickness) (Tc) is Tc≦0.12 mm, an outer wall thickness (Ts) of thehoneycomb structure is Ts≧0.05 mm, and an open frontal area (P) isP≧80%, and there is a relation shown by a formula:

1.10≦(Tr ₁ ˜Tr ₃₋₂₀)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₂₀) of cells existing between an outermost peripheral cell andany cell within a first end cell from a third cell to a twentieth cellextending inwardly, taking the outermost peripheral cell as a firststarting cell.

As described above, in the ceramic honeycomb structure of the presentinvention, the basic cell wall thickness (Tc) of the cell wallsconstituting the honeycomb structure is 0.12 mm or less, preferably 0.07mm or less; the honeycomb outer wall thickness (Ts) is 0.05 mm or more,preferably 0.1 mm or more; the open frontal area (P) of_the basic cellportion is 80% or more; and a relation of 1.10≦(Tr₁˜Tr₃₋₁₅)/Tc≦3.00,preferably 1.10≦(Tr₁˜Tr₃₋₁₅)/Tc≦2.50, more preferably1.20≦(Tr₁˜Tr₃₋₁₅)/Tc≦1.60 is allowed to hold between the basic cell wallthickness (Tc) and each cell wall thickness (Tr₁˜Tr₃₋₁₅) of the cellsexisting between each outermost peripheral cell (first starting cell)and any cell (first end cell) of the 3rd to 15th cells extendinginwardly from the first starting cell. Thereby, a thin wall honeycombstructure can be obtained wherein the disadvantage incurred by theincreased pressure loss is balanced against the advantage brought aboutby the increased isostatic strength are appropriately balanced, andwhich has an increased isostatic strength and a cell wall shape andhoneycomb external shape of higher accuracy.

The embodiment of the ceramic honeycomb structure of the presentinvention is described below more specifically with referring to theaccompanying drawings.

FIG. 1(a) is a perspective view schematically showing an example of theceramic honeycomb structure of the present invention, and FIG. 1(b) is aplan view thereof. A ceramic honeycomb structure 1 comprises a pluralityof parallel passages (cells) 3 separated by cell walls 2. Each outermostperipheral cell of the plurality of cells 3 is surrounded and held by ahoneycomb outer wall 4.

FIG. 2(a) is a partly enlarged view of the portion A of FIG. 1(b), andFIG. 2(b) is a further enlarged view of FIG. 2(a). As shown in FIGS.2(a) and 2(b), there is each outermost peripheral cell (first startingcell) 8 in the nearest vicinity of the outer wall 4; and a second cell 9extends inwardly from the outermost peripheral cell (first startingcell) 8. The wall thickness of the outermost peripheral cell (firststarting cell) 8 is shown by Tr₁, and the wall thickness of the secondcell 9 is shown by Tr₂. Similarly, the wall thickness of any cell of thecells existing between 3rd to 15th cells is shown by Tr₃₋₁₅.Incidentally, the cell walls 2 are largely divided into walls 2 a ofcells near the circumference of honeycomb and walls 2 b of basic cells.

In the ceramic honeycomb structure of the present invention, thefollowing relation is specified

1.10≦(Tr ₁ ˜Tr ₃₋₁₅)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₁₅) of the cells existing between each outermost peripheralcell taken as starting cell and any cell taken as end cell of the 3rd to15th cells extending inwardly from the starting cell.

When the value [(Tr₁˜Tr₃₋₁₅)/Tc] is less than 1.10, no improvement inisostatic strength is seen. When the value is more than 3.00, pressureloss increases.

Thus, in the honeycomb structure 1 of the present invention, each cellwall thickness (Tr₁˜Tr₃₋₁₅) of the cells existing between each outermostperipheral cell (1st cell) and any cell of the 3rd to 15th cellsextending inwardly from the 1st cell is made larger than the basic cellwall thickness (Tc) by a given proportion. However, when only the wallthickness (Tr₁˜Tr₂) of the 1st to 2nd cells is made larger by a givenproportion, there is no improvement in isostatic strength or externalform accuracy (cell wall shape accuracy); and when the wall thickness ofthe cells existing between the 1st cell and any cell located inward fromthe 15th cell is made larger by a given proportion, pressure lossincreases; moreover, carrier mass increases to a level higher thanspecified, resulting in increased heat capacity, which may adverselyaffect the warm-up property of catalyst during cold start.

In the ceramic honeycomb structure of the present invention, it ispreferred that, as shown in FIG. 3(a) to FIG. 3(c), the section of eachcell wall, within the 2nd end cell of from the 3rd to 5th cellsextending inwardly, taking the cell adjacent to the 1st end cell butlocated inward therefrom as the 2nd starting cell is cut by a planeperpendicular to the cell (passage) direction, is such a rectangularshape as the minor side is the thickness of said each cell wall [FIG.3(c)], or such an inverse trapezoidal shape as the minor base of inversetrapezoid is present inwardly and is the thickness of said each cellwall [FIG. 3(a)], or such a spool shape as the inner side of spool isshorter than the outer side and is the thickness of said each cell wall[FIG. 3(b)]; the minor side of rectangle, or the inward minor base ofinverse trapezoid or the inner side of spool is made shorter as saideach cell wall is more inward (the degree of shortening may be 1.10 to3.00); and the thickness of the cell wall having the shortest minorside, or the shortest minor base or the shortest inner side is madeidentical to the basic cell wall thickness (Tc). By constituting thepresent honeycomb structure as above, improvements in pressure loss andthermal shock resistance may be obtained.

In order to prevent the increase in the weight of honeycomb carrier,caused by the thickening of the cell walls near the circumference ofhoneycomb structure, it is possible to apply padding to the internalportion of the outer wall on at least the areas where outermostperipheral cell walls located adjacent each other come into contact withthe outer wall of honeycomb structure, with making the spacetherebetween narrower (this padding is contact padding) or to theV-shaped areas formed by contact between each intersection of two cellwalls and the outer wall (this padding is V-shaped padding) and therebyachieve a rib shape of higher accuracy, an improved isostatic strength,etc. and make the cell wall thickness relatively thin.

Specifically, the corners of each cell are formed so as to have a radiusof curvature of preferably 1.2 mm or less and the intersection betweencell wall and honeycomb outer wall is formed so as to have a radius ofcurvature of preferably 1.2 mm or less.

In the present invention, it is also preferred that the followingrelation is allowed to hold

1.10≦Tr ₁ /Tc≦3.00

between the cell wall thickness (Tr₁) of each outermost peripheral celland the basic cell wall thickness (Tc),

when the outermost peripheral cell is referred to as third starting celland any of the 3rd to 20th cells extending inwardly from the thirdstarting cell is referred to as third end cell, the following relationis allowed to hold

1.10≦(Tr ₁ ˜Tr ₃₋₂₀)/Tc≦3.00

between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₂₀) of the cells existing between the third starting cell andthe third end cell, and

when the honeycomb structure is cut by a plane perpendicular to thedirection of the cells (passages) and the resulting section of each cellwall of the cells existing from the third starting cell to the third endcell is seen, the section of said each cell wall is allowed to have sucha rectangular shape as the minor side is the thickness of said each cellwall, or such an inverse trapezoidal shape as the minor base of inversetrapezoid is present inwardly and is the thickness of said each cellwall, or such a spool shape as the inner side of spool is shorter thanthe outer side and is the thickness of said each cell wall; the minorside of rectangle, or the inward minor base of inverse trapezoid or theinner side of spool is made shorter as said each cell wall is moreinward; and the thickness of the cell wall having the shortest minorside, or the shortest minor base or the shortest inner side is madeidentical to the basic cell wall thickness (Tc). By constituting thepresent honeycomb structure as above, improvements in pressure loss andthermal shock resistance can be obtained.

In the present invention, it is also preferred in view of the pressureloss in practical application that the basic cell wall thickness andeach cell wall thickness (Tr₁˜Tr₃₋₂₀) have, as mentioned previously, amore restricted relation of 1.10≦(Tr₁˜Tr₃₋₂₀)/Tc≦2.50, or an even morerestricted relation of 1.20≦(Tr₁˜Tr₃₋₂₀)/Tc≦1.60.

As to the sectional shape of cell, used in the present invention, thereis no particular restriction. However, a sectional shape of, forexample, a triangle or a higher polygon can be mentioned. In particular,any of a square, rectangle and a hexagon is preferred.

As the sectional shape of honeycomb outer wall, used in the presentinvention, there can be mentioned, for example, a circle, an ellipse, atrapezoid, a triangle, a square, a hexagon or a special shape whose leftand right are asymmetrical to each other. Of these, a circle or anellipse is preferred.

In recent years, honeycomb carriers have come to be mounted in largevehicles (e.g. trucks) as well and large-sized honeycomb carriers havebecome necessary. In the case of such a large-sized honeycomb carrier ashaving a diameter of 144 mm or more when the sectional shape ofhoneycomb outer wall is a circle, or, when the sectional shape is othershape, having a sectional area equal to when the sectional shape is acircle, it is preferred that the first end cell counted from theoutermost peripheral cell (1st starting cell) is stretched to any of the10th to 40th cells, preferably the 10th to 30th cells, all extendinginwardly from the outermost peripheral cell, that is, the cells ofthickened wall are increased and that the ratio of the cell wallthickness (Tr₁˜Tr₁₀₋₄₀), preferably (Tr₁˜Tr₁₀₋₃₀) to the basic cell wallthickness (Tc), i.e. (Tr₁˜Tr₁₀₋₄₀)/Tc, preferably (Tr₁˜Tr₁₀₋₃₀)/Tc isset ordinarily at 1.10 to 3.00 and, in practical application, at 1.10 to2.50, preferably 1.20 to 1.60.

As the material for cell wall and honeycomb outer wall, used in thepresent invention, there can be mentioned, for example, at least onekind of material selected from the group consisting of cordierite,alumina, mullite, silicon nitride, aluminum titanate (AT), zirconia andsilicon carbide.

FIG. 4 is a drawing schematically showing a case in which the honeycombcarrier of the present invention has been accommodated in a catalyticconverter container. A honeycomb carrier 13 is held by a ring 12 at theouter surface and accommodated in a converter container 11. There is noparticular restriction as to the ring 12, but a metal mesh-made ring isordinarily used. Between the converter container 11 and the outersurface of the honeycomb carrier 13, a buffer member 14 (e.g. a mat or acloth) is preferably interposed.

Next, the present invention is more specifically described by way ofExamples. However, the present invention is in no way restricted bythese Examples.

Incidentally, the honeycomb structures obtained in the Examples wereevaluated for performance by the following methods.

Isostatic Strength Test

The test and evaluation were made according to the JASO standard M505-87 issued by Society of Automotive Engineer of Japan, Inc. In Table1, evaluation was made by three grades of no increase (increase was notseen as a significance difference as compared with standard), smallincrease, and increase.

Thermal Shock Resistance Test

This is a test in which a honeycomb carrier of room temperature isplaced in an electric furnace kept at a temperature higher than roomtemperature by a given temperature, is kept for 20 minutes, is taken outonto a refractory brick, is observed for appearance, and is lightlytapped by a metal bar at the outer surface. An evaluation of “pass” isgiven when the carrier appearance shows no crack and a metallic sound(not a thick sound) is heard when tapped. The test is repeated until a“fail” evaluation is reached when the temperature inside the electricfurnace is gradually increased by each 50° C. When “fail” is reached ata temperature of 950° C. higher than room temperature, the thermal shockresistance of the honeycomb carrier is taken as 900° C. difference.

External Shape Accuracy Test

Accuracy of honeycomb structure circumference was measured using athree-dimension tester.

Pressure Loss Test

A converter accommodating a catalyst-loaded honeycomb structure wasfitted to a 2-liter 4-cylinder engine. A difference between the pressureat the converter inlet and the pressure at the converter outlet wasmeasured and taken as the pressure difference of the honeycombstructure.

EXAMPLES 1 TO 53 AND COMPARATIVE EXAMPLES 1 TO 25

A kneaded raw material consisting of talc, kaolin, alumina, water and abinder was subjected to extrusion molding and then fired to producevarious cordierite-based honeycomb structure samples each having asquare cell shape, a diameter of 106 mm, a length of 155 mm, an outerwall thickness of 0.2 mm and an open frontal area (P) of 80% or more. Inthese samples, the cell structure, basic cell wall thickness, No. ofcells of thickened wall near the circumference of honeycomb structure,thickness of thickened cell walls near the circumference of honeycombstructure, and ratio of thickness of thickened cell walls to basic cellwall thickness were varied as shown in Table 1. Each honeycomb structure(carrier) produced was subjected to isostatic strength test, thermalshock resistance test, pressure loss test and external shape accuracytest. The results are shown in Tables 1 to 4.

Incidentally, Example 18 is a case where the wall thickness of theoutermost peripheral cell (starting cell) to the 15th cell was set at0.150 mm and each wall of the 16th to 20th cells was formed as follows:

the section of said each wall had such an inverse trapezoidal shape asthe minor base of inverse trapezoid was present inwardly, the minor baseof inverse trapezoid was shorter as said each wall was more inward, andthe thickness of the cell wall having the shortest minor base wasidentical to the basic cell wall thickness (Tc) (0.075 mm). Example 19is a case where the wall thickness of the outermost peripheral cell(starting cell) to the 15th cell was set at 0.150 mm and each wall ofthe 16th to 20th cells was formed as follows:

the section of said each wall had such a spool shape as the inner sideof spool was shorter than the outer side, the inner side of spool wasshorter as said each wall was more inward, and the thickness of the cellwall having the shortest inner side was identical to the basic cell wallthickness (Tc) (0.075 mm). Examples 20 and 21 are cases where each wallof the outermost peripheral cell (staring cell) to the 20th cell wasformed as follows:

the section of said each wall had an inverse trapezoidal or spool shape,the thickness of said each wall was shorter as said each wall was moreinward, and the thickness of the cell wall having the smallest thicknesswas identical to the basic cell wall thickness (Tc) (0.075 mm). The samething is also applicable to Examples 50, 51, 52 and 53.

TABLE 1 Basic cell Thickness No. of Example or Cell wall of thicke-cells of Pressure Comparative Structure thickness ned cell thickenedIsostatic Shape loss Thermal shock Example (mil/cpsi) (A) walls (B) (B)/ (A) wall strength accuracy (hp) resistance Comparative 3.0/600 0.0750.075 1.0 0 Standard Standard Standard Standard example 1 Comparative3.0/600 0.075 0.100 1.33 2 No increase Not Small Equivalent to example 2improved increase standard Example 1 3.0/600 0.075 0.100 1.33 5 SmallNot Small Equivalent to increase improved increase standard Example 23.0/600 0.075 0.100 1.33 8 Increase Slightly Increase Equivalent toimproved standard Example 3 3.0/600 0.075 0.100 1.33 11 IncreaseImproved Increase Equivalent to standard Comparative 3.0/600 0.075 0.0801.07 13 No increase Slightly Increase Equivalent to example 3 improvedstandard Example 4 3.0/600 0.075 0.085 1.13 13 Small Slightly IncreaseEquivalent to increase improved standard Example 5 3.0/600 0.075 0.0901.20 13 Small Improved Increase Equivalent to increase standard Example6 3.0/600 0.075 0.100 1.33 13 Small Improved Increase Equivalent toincrease standard Comparative 3.0/600 0.075 0.150 2.00 2 No increase NotSmall — example 4 improved increase Example 7 3.0/600 0.075 0.150 2.00 5Small Slightly Increase — increase improved Example 8 3.0/600 0.0750.150 2.00 8 Increase Improved Increase — Example 9 3.0/600 0.075 0.1502.00 11 Increase Improved Increase — Example 10 3.0/600 0.075 0.150 2.0013 Increase Improved Increase — Example 11 3.0/600 0.075 0.200 2.67 2 Noincrease Not — — improved Example 12 3.0/600 0.075 0.200 2.67 5 SmallSlightly — — increase improved Example 13 3.0/600 0.075 0.200 2.67 8Increase Improved — — Example 14 3.0/600 0.075 0.200 2.67 11 IncreaseImproved — — Example 15 3.0/600 0.075 200     67 13 Increase ImprovedIncrease — Example 16 3.0/600 0.075 0.225 3.00 13 Increase — Increase —Comparative 3.0/600 0.075 0.240 3.20 13 Increase — Big — Example 5increase Comparative 3.0/600 0.075 0.260 3.50 13 Increase — Big —example 6 increase Comparative 3.0/600 0.075 0.300 4.00 13 Increase —Big — example 7 increase Example 17 3.0/600 0.075 0.150 2.00 15 Increase— Increase Decrease Example 18 3.0/600 0.075    0.150 ˜ 2.00 15 Increase— Increase Slight 0.075 Inverse 16-20 decrease trapezoid- like decreaseExample 19 3.0/600 0.075    0.150 ˜ 2.00 15 Increase — Increase Slight0.075 Spool-like 16-20 decrease decrease Example 20 3.0/600 0.075   0.150 ˜ Inverse 20 Increase Improved Smaller Equivalent to 0.075trapezoid- than in standard like Exams. 17 decrease to 19 Example 213.0/600 0.075    0.150 ˜ Spool-like 20 Increase Improved SmallerEquivalent to 0.075 decrease than in standard Exams. 17 to 19 Example 223.0/600 0.075 0.150 2.00 10 Increase — Increase — Example 23 3.0/6000.075 0.150 2.00 20 — — Increase — Example 24 3.0/600 0.075 0.150 2.0025 — — Big — increase Example 25 3.0/600 0.075 0.150 2.00 30 — — Big —increase

TABLE 2 Basic cell Thickness No. of Example or Cell wall of thicke-cells of Pressure Comparative Structure thickness ned cell thickenedIsostatic Shape loss Thermal shock Example (mil/cpsi) (A) walls (B) (B)/ (A) wall strength accuracy (hp) resistance Comparative 2.0/900 0.0500.050 1.0 0 Standard Standard Standard Standard Example 8 Example 262.0/900 0.050 0.055 1.1 10 Small increase Slightly Increase Equivalentto standard improved Example 27 2.0/900 0.050 0.060 1.2 10 Smallincrease Improved Increase Equivalent to standard Example 28 2.0/9000.050 0.065 1.3 10 Small increase Improved Increase Equivalent tostandard Example 29 2.0/900 0.050 0.070 1.4 10 Increase ImprovedIncrease Equivalent to standard Example 30 2.0/900 0.050 0.075 1.5 10Increase Improved Increase Equivalent to standard Example 31 2.0/9000.050 0.080 1.6 10 Increase Improved Increase Equivalent to standardExample 32 2.0/900 0.050 0.085 1.7 10 Increase Improved Increase Slightdecrease Example 33 2.0/900 0.050 0.090 1.8 10 Increase ImprovedIncrease Slight decrease Example 34 2.0/900 0.050 0.100 2.0 10 IncreaseImproved Increase Decrease Example 35 2.0/900 0.050 0.125 2.5 10Increase Improved Increase Decrease Example 36 2.0/900 0.050 0.150 3.010 Increase Improved Increase Slight decrease Comparative 2.0/900 0.0500.175 3.5 10 Increase Improved Big increase Big decrease Example 9Example 37 2.0/900 0.050 0.080 1.6 2 No increase Not Small Equivalent tostandard improved increase Example 38 2.0/900 0.050 0.080 1.6 5 Smallincrease Slightly Small Equivalent to standard improved increase Example39 2.0/900 0.050 0.080 1.6 7 Small increase Improved Increase Equivalentto standard Example 40 2.0/900 0.050 0.080 1.6 10 Increase ImprovedIncrease Equivalent to standard Example 41 2.0/900 0.050 0.080 1.6 15Increase Improved Increase Slight decrease Example 42 2.0/900 0.0500.080 1.6 20 Increase — Increase Slight decrease Comparative 2.0/9000.050 0.080 1.6 25 Increase — Big increase Slight decrease Example 10

TABLE 3 Basic cell Thickness No. of Example or Cell wall of thicke-cells of Pressure Comparative Structure thickness ned cell thickenedIsostatic Shape loss Thermal shock Example (mil/cpsi) (A) walls (B) (B)/ (A) wall strength accuracy (hp) resistance Comparative 5.0/200 0.1250.125 1.0 0 Standard Standard Standard Standard Example 11 Comparative5.0/200 0.125 0.200 1.6 10 Increase Improved Increase Equivalent toExample 12 standard Comparative 4.5/300 0.115 0.115 1.0 0 StandardStandard Standard Standard Example 13 Example 43 4.5/400 0.115 0.1751.52 10 Increase Improved Increase Equivalent to standard Comparative4.0/400 0.100 0.100 1.0 0 Standard Standard Standard Standard Example 14Example 44 4.0/400 0.100 0.150 1.5 10 Increase Improved IncreaseEquivalent to standard Comparative 3.5/400 0.090 0.090 1.0 0 StandardStandard Standard Standard Example 15 Comparative 3.5/400 0.090 0.1351.50 2 No increase Not improved Small increase Equivalent to Example 16standard Example 45 3.5/400 0.090 0.135 1.50 10 Increase ImprovedIncrease Equivalent to standard Example 46 3.5/400 0.090 0.135 1.50 15Increase Improved Increase Equivalent to standard Comparative 3.5/4000.090 0.135 1.50 25 Increase Improved Big increase Slight decreaseExample 17 Comparative 3.5/600 0.090 0.090 1.0 0 Standard StandardStandard Standard Example 18 Example 47 3.5/600 0.090 0.125 1.39 10Increase Improved Increase Equivalent to standard

TABLE 4 Basic cell Thickness No. of Example or Cell wall of thicke-cells of Pressure Comparative Structure thickness ned cell thickenedIsostatic Shape loss Thermal shock Example (mil/cpsi) (A) walls (B) (B)/ (A) wall strength accuracy (hp) resistance Comparative 1.5/900  0.0350.035 1.0 0 Standard Standard Standard Standard Example 19 Example 481.5/900  0.035 0.065 1.86 15 Increase Improved Increase Equivalent tostandard Comparative 2.0/1200 0.050 0.050 1.0 0 Standard StandardStandard Standard Example 20 Example 49 2.0/1200 0.050 0.080 1.60 10Increase Improved Increase Equivalent to standard Comparative 1.5/12000.035 0.035 1.0 0 Standard Standard Standard Standard Example 21Comparative 1.5/1200 0.035 0.065 1.86 2 No increase Not Small increaseEquivalent to Example 22 improved standard Comparative 1.5/1200 0.0350.065 1.86 10 Increase Improved Increase Equivalent to Example 23standard Example 50 1.5/1200 0.035     0.065 ˜ 1.86 15 Increase ImprovedIncrease Equivalent to 0.035 Inverse trapezoid- 16-20 standard likedecrease Example 51 1.5/1200 0.035    0.065 ˜ Inverse trapezoid- 30Increase Improved Big increase Slight decrease 0.035 like decreaseComparative 1.0/1200 0.025 0.035 1.0 0 Standard Standard StandardStandard Example 24 Example 52 1.0/1200 0.025    0.065 ˜ 2.6 10 IncreaseImproved Increase Equivalent to 0.025 Inverse trapezoid- 11-20 standardlike decrease Comparative 1.0/1800 0.025 0.035 1.0 0 Standard StandardStandard Standard Example 25 Example 53 1.0/1800 0.025    0.065 ˜ 2.6 10Increase Improved Increase Equivalent to 0.025 Inverse trapezoid- 11-20standard like decrease

As appreciated from Tables 1 to 4, good results are obtained when therequirements for the present invention are satisfied. Similar resultswere obtained also with other cell structures of triangle, hexagon, etc.

The above performance evaluation of honeycomb structures was made on acase of no cell deformation in honeycomb structure. It was confirmedthat similar performances are obtained also on a case of celldeformation in honeycomb structure, as long as there is a relation of1.10≦(Tr₁˜Tr₃₋₂₀)/Tc≦3.00 between the basic cell wall thickness (Tc) andeach cell wall thickness (Tr₁˜Tr₃₋₂₀).

Herein, “cell deformation” refers to a state in which cell wall (rib) isdeformed relative to the central axis of honeycomb carrier. The amountof deformation indicates a case in which deformation is 1.1 to 5.0 timesthe thickness of cell wall (rib).

It was confirmed that in a ceramic honeycomb structure having “celldeformation”, a sufficient strength can be obtained when the 1st or 3rdend cell is any of the 3rd to 5th cells (when the honeycomb structurehas a diameter of 120 mm or less) or the 1st or 3rd end cell is any ofthe 6th to 20th cells (when the honeycomb structure has a diameter ofmore than 120 mm). That is, as shown in FIG. 16, when the honeycombstructure had a diameter of 120 mm or less and the 1st or 3rd end cellwas any of the 3rd to 5th cells, there was obtained a sufficientstrength which was 2 times as compared with a honeycomb structure usingno reinforced cell and having an ordinary cell wall thickness; when thehoneycomb structure had a diameter of more than 120 mm and the 1st or3rd end cell was any of the 6th to 20th cells, there was obtained asufficient strength which was 2 times as compared with a honeycombstructure using no reinforced cell and having an ordinary cell wallthickness. Even when the honeycomb structure had a diameter of more than120 mm and the 1st or 3rd end cell was any of the 3rd to 5th cells,there was obtained a sufficient strength which was 1.2 times as comparedwith a honeycomb structure using no reinforced cell and having anordinary cell wall thickness.

It was also confirmed that a honeycomb structure having, in the cellsfrom the starting cell to the end cell, a corrugated cell wall (such asshown in FIG. 17) having a corrugation at the intersection of at leastone pair of two adjacent walls (for example, a pair of Rx and Ry in FIG.17) was superior, in thermal shock resistance, to a honeycomb structurehaving an ordinary cell wall having no corrugation, in addition to theabove-mentioned evaluated performance. That is, it is confirmed that ahoneycomb structure having a corrugated cell wall showed a superiorthermal shock resistance (a higher resistance to cracking) than ahoneycomb structure having an ordinary cell wall having no corrugationby subjecting the place including the boundary portion possessing atleast corrugated shape to a cycle of heating at 1,200° C. for 5 minutesand then cooling for 5 minutes was conducted 10 times (10 cycles)according to a burner test (a test for examining the tendency of crackformation) described later. The honeycomb structures used in this testhad the same material and shape as in Example 1.

Burner Test

A tester shown in FIG. 18 was used. This tester is Maremont Exhaust GasSimulator Model No. 3 or an equivalent thereto, or a hydraulic isostatictester (a product of NGK INSULATORS, LTD.) or an equivalent thereto. Asshown in FIG. 18, this tester has a main burner 101, a pilot burner 102,a combustion chamber 103, a bypass 104 and a holder 105. Into thecombustion chamber 103 are fed an LPG 106 and heating air 107, andignition is made using a spark plug 108 to give rise to combustion. Inthe holder 105 are set a sample 110 and, in the vicinity thereof, athermocouple 109. The bypass 104 switches the heating air 107 andcooling air 111 to each other, whereby a cold-hot cycle is applied tothe sample 110.

The test method was as follows. First, a holding material (not shown inFIG. 18) was wound round the sample 110, and the sample was set in theholder 105 so that no gas passed outside the sample 110. Then, thethermocouple 109 was set 5 mm upstream of the gas inlet end of thesample 110. Then, a cold-hot cycle of 1,200° C.×10 cycles, shown in FIG.19 was applied to the sample 110. Lastly, formation of cracks in thesample 110 was examined by observation. Incidentally, the gas flow rateduring heating was 1.0 Nm³/min.

Industrial Applicability

The honeycomb structure of the present invention can be suitably used asa carrier for catalyst, etc., particularly as a carrier for catalyst forautomobile exhaust gas purification, etc. The honeycomb structure of thepresent invention is also used suitably as a filter for dieselparticulate or the like, as a chemical reactor (e.g. a catalyst carrierfor fuel cell reformer), or as a heat exchanger.

What is claimed is:
 1. A ceramic honeycomb structure (1) constituted bycell walls (ribs) (2) forming a composite structure from a plurality ofcells (3) being adjacent each other and a honeycomb outer wall (4)forming a circumference of the composite structure surrounding andholding the outermost peripheral cells located at the circumference ofthe composite structure the contact the outer wall; characterized inthat a basic thickness of cell walls (2) (the basic cell wall thickness)(Tc) is Tc≦0.12 mm, an outer wall thickness (Ts) of the honeycombstructure is Ts≧0.05 mm, and an open frontal area (P) is P≧80%, andthere is a relation shown by a formula: 1.10≦(Tr ₁˜Tr₃₋₂₀)/Tc≦3.00between the basic cell wall thickness (Tc) and each cell wall thickness(Tr₁˜Tr₃₋₂₀) of cells existing between an outermost peripheral cell andany cell within a first end cell from a third cell to a twentieth cellextending inwardly, taking the outermost peripheral cell as a firststarting cell.
 2. A ceramic honeycomb structure according to claim 1,wherein there is a relation shown by a formula: 1.10≦(Tr ₁ ˜Tr₃₋₁₅)/Tc≦3.00 between the basic cell wall thickness (Tc) and each cellwall thickness (Tr₁˜Tr₃₋₁₅) of cells existing between an outermostperipheral cell and any cell within a first end cell from a third cellto a fifteenth cell extending inwardly, taking the outermost peripheralcell as a first starting cell.
 3. A ceramic honeycomb structureaccording to claim 1, wherein any cell within a second end cell from athird cell to a fifth cell extending inwardly, taking a cell adjacent tothe first end cell but located inward therefrom as a second startingcell, has such a cell wall thickness that a section of said each cellwall has a rectangular shape whose minor side of rectangle is a cellwall thickness thereof when the honeycomb structure is cut by a planeperpendicular to the direction of the cells (passages), and a cell wallthickness having a shortest minor side is identical to the basic cellwall thickness (Tc), by shortening a minor side thereof one by one as acell is located more inwardly.
 4. A ceramic honeycomb structureaccording to claim 1, wherein any cell within a second end cell from athird cell to a fifth cell extending inwardly, taking a cell adjacent tothe first end cell but located inward therefrom as a second startingcell, has such a cell wall thickness that a section of said each cellwall has such an inverse trapezoidal shape as a minor base of inversetrapezoid is a thickness of said each cell wall when the honeycombstructure is cut by a plane perpendicular to the direction of the cells(passages), and a thickness of a cell wall having a shortest minor baseis identical to the basic cell wall thickness (Tc), by shortening aminor base of inverse trapezoid thereof one by one as said each cellwall is located more inwardly.
 5. A ceramic honeycomb structureaccording to claim 1, wherein any cell within a second end cell from athird cell to a fifth cell extending inwardly, taking a cell adjacent tothe first end cell but located inward therefrom as a second startingcell, has such a cell wall thickness that a section of said each cellwall has such a spool shape as an inner side of spool is shorter than anouter side when the honeycomb structure is cut by a plane perpendicularto the direction of the cells (passages), and a thickness of a cell wallhaving an shortest inner side is identical to the basic cell wallthickness (Tc), by shortening inner side of spool thereof one by one assaid each cell wall is located more inwardly.
 6. A ceramic honeycombstructure according to claim 1, wherein there is a relation shown by aformula 1.10≦Tr ₁ /Tc≦3.00 between the cell wall thickness (Tr₁) of eachoutermost peripheral cell and the basic cell wall thickness (Tc), thereis a relation shown by a formula 1.10≦(Tr ₁˜Tr₃₋₂₀)/Tc≦3.00 between thebasic cell wall thickness (Tc) and each cell wall thickness (Tr₁˜Tr₃₋₂₀)within a third end cell from a third cell to a twentieth cell extendinginwardly, taking the outermost peripheral cell as a first starting cell,a section of said each cell wall has such a rectangular shape as a minorside thereof is thickness of said each cell wall, or such an inversetrapezoidal shape as a minor base of inverse trapezoid is presentinwardly and is thickness of said each cell wall, or such a spool shapeas inner side of spool is shorter than outer side when the honeycombstructure is cut by a plane perpendicular to the direction of the cells(passages); and a thickness of the cell wall having a shortest minorside, or a shortest minor base or a shortest inner side is identical tothe basic cell wall thickness (Tc), by shortening the minor side ofrectangle, or the inward minor base of inverse trapezoid or the innerside of spool one by one as said each cell wall is located moreinwardly.
 7. A ceramic honeycomb structure according to claim 1, whereinthere is a relation shown by a formula: 1.10≦(Tr ₁ ˜Tr ₃₋₂₀)/Tc≦2.50between the basic cell wall thickness (Tc) and said each cell wallthickness (Tr₁˜Tr₃₋₂₀).
 8. A ceramic honeycomb structure according toclaim 1, wherein there is a relation shown by a formula: 1.20≦(Tr ₁ ˜Tr₃₋₂₀)/Tc≦1.60 between the basic cell wall thickness (Tc) and said eachcell wall thickness (Tr₁˜Tr₃₋₂₀).
 9. A ceramic honeycomb structureaccording to claim 1, wherein the cells have a sectional shape of atriangle or a higher polygon.
 10. A ceramic honeycomb structureaccording to claim 1, wherein the honeycomb outer wall has a sectionalshape of a circle, an ellipse, a trapezoid, a triangle, a tetragon, ahexagon or a special shape whose left and right are asymmetrical to eachother.
 11. A ceramic honeycomb structure according to claim 1, whereinthe honeycomb outer wall has a diameter of 144 mm or more when it has acircular sectional shape and, when it has a sectional shape other than acircular sectional shape, it has a sectional area equal to when it has acircular sectional shape, and, there is the following relation 1.10≦(Tr₁˜Tr₁₀₋₄₀)/Tc≦3.00 between the basic cell wall thickness (Tc) and eachcell wall thickness (Tr₁˜Tr₁₀₋₄₀) of cells existing within a first endcell from a third cell to a fortieth cell extending inwardly, taking theoutermost peripheral cell as a first starting cell.
 12. A ceramichoneycomb structure according to claim 1, wherein the honeycomb outerwall has a diameter of 144 mm or more when it has a circular sectionalshape and, when it has other than a circular sectional shape, it has asectional area equal to when it has a circular sectional shape, and,there is a following relation shown by a formula: 1.10≦(Tr ₁ ˜Tr₁₀₋₃₀)/Tc≦3.00 between the basic cell wall thickness (Tc) and each cellwall thickness (Tr₁˜Tr₁₀₋₃₀) of cells within a first starting end cellfrom a tenth cell to a thirtieth cells extending inwardly, taking theoutermost peripheral cell as a first starting cell.
 13. A ceramichoneycomb structure according to claim 1, wherein the cell walls and thehoneycomb outer wall are made of at least one kind of materials selectedfrom the group consisting of cordierite, alumina, mullite, siliconnitride, aluminum titanate (AT), zirconia and silicon carbide.
 14. Aceramic honeycomb structure according to claim 1, which is used as acarrier for catalyst for automobile exhaust gas purification.
 15. Aceramic honeycomb structure according to claim 1, which is assembledinto a catalytic converter by loading a catalyst component on cell wallsand holding honeycomb outer wall at outer surface.
 16. A ceramichoneycomb structure according to claim 1, wherein corners of each cellare formed so as to have a radius of curvature of 1.2 mm or less.
 17. Aceramic honeycomb structure according to claim 1, wherein eachintersection between each outermost peripheral cell wall and thehoneycomb outer wall is formed so as to have a radius of curvature of1.2 mm or less.
 18. A ceramic honeycomb structure according to claim 1,wherein there is cell deformation and, when a diameter of the honeycombstructure is 120 mm or less, a first or third end cell is any of a thirdcell to a fifth cell and, when a diameter is more than 120 mm, a firstor a third end cell is any of a sixth cell to a twentieth cell.
 19. Aceramic honeycomb structure according to claim 1, wherein there isprovided with a corrugated cell wall having a corrugation in thedirection of the cells (passages) between at least one pair of cellsadjacent to each other, of the cells from the first starting cell to thefirst end cell or from the second starting cell to the second end cellor from the third starting cell to the third end cell.